The present invention relates to an air-fuel ratio controller for an internal-combustion engine, which responds to outputs from two oxygen sensors installed in an exhaust system of the engine, and more particularly, to an air-fuel ratio controller operable to carry out a proper air-fuel ratio control in response to the output of a downstream-side oxygen sensor, to thereby improve the exhaust gas performance of an internal-combustion engine.
An air-fuel ratio controller of a so-called dual O.sub.2 sensor type operable to control the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine in accordance with output signals from two oxygen sensors respectively installed on the upstream and downstream sides of an exhaust gas purifier, e.g., a three way catalyst, which is disposed in an exhaust passage of the engine, is known through, e.g., Japanese Provisional Patent Publication No. 64-53043 corresponding to U.S. Pat. No. 4,912,926. In the air-fuel ratio controller of this kind, the air-fuel ratio is adjusted to be lean if an output value of the oxygen sensor (hereinafter referred to as a "front O.sub.2 sensor) provided on the upstream side of the three way catalyst is greater than a first reference discriminant value, while the air-fuel ratio is adjusted to be rich when the output value of the front O.sub.2 sensor is smaller than the first reference discriminant value. In addition, the first reference discriminant value is feedback-corrected to a value which, for instance, allows an optimum exhaust gas characteristic to be achieved, in accordance with the deviation of the output value of the oxygen sensor (called a "rear O.sub.2 sensor") installed on the downstream side of the three way catalyst from a second reference discriminant value.
In addition to the aforesaid method of feedback controlling the first reference discriminant value to an optimum value in accordance with the deviation of the rear O.sub.2 sensor output from the second reference discriminant value, a method is known in which the integral gain or proportional gain in the air-fuel ratio control carried out based on the output signal value of the front O.sub.2 sensor is feedback-corrected to an optimum value In accordance with the deviation of the output value of the rear O.sub.2 sensor from the second reference discriminant value. It is also well known that, in the similar air-fuel ratio control, the period from the moment the output value of the front O.sub.2 sensor crosses the first reference discriminant value to the moment when the air-fuel ratio is corrected (i.e., the delay period of the air-fuel ratio correction implementing timing) is positively feedback-corrected according to the deviation between the rear O.sub.2 sensor and the second reference discriminant value, thereby improving the exhaust gas characteristics.
When a parameter such as the first reference discriminant value, integral gain, proportional gain or delay period is corrected in accordance with the deviation between the rear O.sub.2 sensor and the second reference discriminant value-as described above, a correction amount of the parameter is conventionally set to a value which is equal to the product of the deviation and a constant correction gain, and hence which is proportional to the magnitude of the deviation. In other words, a correction gain, which takes a constant value regardless of the magnitude of the deviation, is used for setting the amount of correction.
However, if the constant correction gain is set to a small value, then the parameter correction amount equal to the product of the magnitude of the deviation and the correction gain value will also take a small value. Therefore, if a significant deviation occurs between the rear O.sub.2 sensor and the second reference discriminant value, then a parameter correction, which is adequate for eliminating the significant deviation, may not be performed. On the other hand, If the correction gain is set to a large value, a significant deviation may cause over-correction of the parameter due to a delayed response of the rear O.sub.2 sensor. Thus, if an excessive or inadequate parameter correction is made, then the degeneration of the exhaust gas performance caused by the deterioration of the O.sub.2 sensor or the catalyst cannot be adequately prevented.